Some considerations when using a microwave oven as a laboratory research tool

Microwave ovens can be used in laboratories for the rapid heating of material - either to dry them completely or to subject a workpiece to sudden thermal stress or electric field stress. Determinations of the moisture content levels in soil or leaf tissue samples, for example, can be made within tens of seconds rather than hours. It is often assumed that placing a load within a microwave oven will result in it being heated evenly as well as quickly, but this is not always the case. This paper describes how a microwave oven works and illustrates how the heating effect within a workpiece can vary. The size and shape of a sample as well as its physical properties determine the power absorption. Equal volumes of water in different shaped containers attain different final temperatures and a tall, cylindrical water load is shown to have different temperatures at different levels. Most microwave ovens do not have a true, variable power capability, but rely on an on/off timing ratio to vary mean power. If this is not appreciated, then erroneous conclusions might be drawn from a set of experiments involving different power levels. Changes in mains supply voltage can affect the amount of energy dissipated in a load. This may introduce variations in results if experiments are conducted over a period of several hours. Experiments are described which illustrate these effects and some criteria and working practices are suggested to improve the consistency and reliability of results when using a microwave oven as a research tool.